1. Field of the Invention
This invention relates generally to a Hybrid Scavengeless Development (HSD) apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a method to prevent toner or other particulate contamination of wires in such an HSD developer unit.
2. Brief Description of Related Developments
Generally, the process of electrophotographic printing includes charging a photoreceptor member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoreceptor surface is exposed to a light image from either a scanning laser beam, an LED source, or an original document being reproduced. This records an electrostatic latent image on the photoreceptor surface. After the electrostatic latent image is recorded on the photoreceptor surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single-component developer material typically comprises toner particles. Toner particles are attracted to the latent image, forming a toner powder image on the photoreceptor surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
Hybrid scavengeless development technology develops toner via a conventional magnetic brush onto the surface of a donor roll. A plurality of electrode wires are closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the electrode wires to generate a toner cloud in the development zone. This donor roll generally consists of a conductive core covered with a thin (50-200 microns) partially conductive layer. The magnetic brush roll is held at an electrical potential difference relative to the donor roll to produce the field necessary for toner to adhere to the donor roll. The toner layer on the donor roll is then disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles. Typical ac voltages of the wires relative to the donor are 700-900 Vpp at frequencies of 5-15 kHz. These ac signals are often square waves, rather than pure sinusoidal waves. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image.
A problem with developer systems using electrode wires is xe2x80x9cWire History.xe2x80x9d Wire history involves highly charged (though sometimes low charged) and generally small toner or other particles being attracted to the wire and sticking to the wire as a result of either adhesive or electrostatic attractive forces. The result is that contaminants build up on the electrodes, as a response to the image area coverage history, causing visible streaks on prints. U.S. Pat. No. 6,049,686 discloses the use of direct current (DC) offset applied to the electrode wires to reduce wire history. It is not practical to routinely work at high direct current (DC) electrode bias offsets because at the same time the offsets improve wire history they reduce the overall level of developability. The electrode DC offset being defined as the DC potential of the electrodes with respect to the magnetic roll DC level. The present invention overcomes the problems of the prior art as will be described in greater detail below.
An image transfer apparatus and a method for removing wire history from the electrodes in a Hybrid Scavengeless Development system.
One embodiment of the invention comprises an image transfer apparatus with a development unit having a development zone containing marking material; an electrode for transporting developing material positioned in the development zone; a donor member that moves in the development zone; a movable imaging member with imaging regions and inter-imaging regions between the imaging regions, the movable imaging member moving both the imaging regions and inter-imaging regions into and out of the development zone; and a voltage supply to electrically bias the electrode, the voltage supply generating a shift relative to nominal in the direct current component of the electrode bias relative to an electrical bias of the donor member during the movement of at least one of the inter-imaging regions through the development zone, wherein the electrode is cleaned.
A second embodiment of the invention comprises an image transfer apparatus, with a development unit having a development zone; a donor member for transporting marking particles to the development zone adjacent an imaging member, the imaging member, having image receiving regions and inter-image areas between the image receiving regions, the imaging member advancing the image receiving regions and the inter-image areas into and out of the development zone; and a voltage supply to electrically bias the donor member relative to the imaging member, the voltage supply generating an electrical bias shift in the donor member from a first electrical bias to a second electrical bias, the electrical bias shift being generated, during the advancement of the inter-image area through the development zone, wherein an electrode in the development zone is cleaned.
A third embodiment of the invention comprises a method of cleaning an image transfer apparatus with the steps of: providing a voltage supply; and supplying voltage from the voltage supply for electrically biasing an electrode with respect to a donor roll; and with the voltage supply, generating a shift in a direct current component of the electrical bias relative to another electrical bias of the donor roll during advancement of an inter-image area.
A fourth embodiment of the invention is a method of transferring an image, with the steps of: generating image regions on an image receiving member, the image regions being separated by inter-image areas; transporting marking particles with a development member to a development zone having an electrode positioned between the image receiving member and the development member; supplying voltage for electrically biasing the development member relative to the image receiving member; and varying at least a direct current component of the electrical bias of the development member to shift at least the direct current component from an initial voltage to another voltage during passage of the inter-image areas through the development zone.